HAZ Toughness Research Articles

Effect of Welding Heat Input on Simulated HAZ Microstructure and Toughness of a V-N Microalloyed Steel

Abstract This study investigates the correlation among the welding heat input, microstructure and impact toughness of the simulated coarse grain heat-affected zone (CGHAZ) of a V-N microalloyed steel which contained 140 ppm of nitrogen using Gleeble simulation technique, light microscope, electron microscope and Charpy-V-Notch (CVN) testing. CVN toughness estimated at −20°C supported that a relatively low energy input is recommended for welding of low carbon V-N microalloyed steel, according to a fact that an impact energy as high as of more than 180j attained in the CGHAZ with heat input less than 25Kj-cm−1, while a CGHAZ with impact energy less than 80j was obtained at a heat input more than 40 Kj-cm−1. Quantitative metallography of samples showed that CVN toughness decreased with the increase of the amount of grain boundary ferrite (GBF). Quantitative metallography of two-stage electrolytically etched samples showed that the averaged size and ratio of lath martensite-austenite (M-A) constituent increased with increasing heat input, while the linear density and amount of M-A decreased. It is indicated that the toughness of CGHAZ was severely dependent on the characteristic parameters of M-A and the amount of GBF. Microhardness of M-A constituents estimated by nanoindentation technique were much higher than that of the neighboring matrix. Severer brittle cracking susceptibility can occur in the CGHAZ with higher heat input based on Chen et al's suggestion that the stress concentration and triaxiality of the neighboring matrix are increased by the hard phase particles such as M-A constituents.

Effect of Niobium on HAZ Microstructure and Toughness of HSLA Steels

Experimental low alloy steels containing different levels of C, Nb and Mn were employed, to each of which a simulated thermal cycle was applied to produce CG-HAZ microstructure. Effect of Nb on the microstructure was observed and it was found that it has some beneficial effects, such as increasing the hardenability, which lead to retardation of GB ferrite and enhancement of intragranular ferrite formation in low and medium carbon steels. For high carbon steels, it enhances the carbides precipitation and as a result the MA area fraction decreases. The initiation site of brittle fracture was found to be dependent on the microstructure. Three categories of microstructure are described; namely grain boundary ferrite with Widmnastatten ferrite plus areas of degenerated pearlite, which is denoted as category I; grain boundary ferrite plus intragranular acicular ferrite which is called category 11; and finally the lath structure which is called category III. The brittle fracture was observed to be initiated at the pearlite colonies in category I, to be associated with the grain boundary ferrite in category II, and to be initiated at the intersection of the bainitic packets in category III. The area fraction ofMA increases by Nb addition for low and medium Carbon steels but for high Carbon steels, MA area fraction shows a slight increase due to carbides precipitation. MA is not likely to play an essential role in the brittle fracture initiation. Mn is the most dominant element affects the formation of MA at the same carbon equivalent. Therefore the beneficial effect ofNb could be obtained through the optimization of alloy design mainly Mn content.

ワンビード溶接法によるHAZ靱性評価法の考察

The engineering method for fracture toughness evaluation of beam-to-column weld HAZ using one-bead welds, called one-bead weld method, has been examined in this study. Materials used are 26 structural steels of 400 to 490 MPa strength class produced by different milling conditions. One-bead welds are made by CO2 arc welding with single bevel groove under heat inputs of 4.0 to 5.0 kJ/mm. From these welded joints, the Charpy specimens and 3-point bend CTOD specimens with the same thickness of 10mm are extracted. The notch is located at the weld HAZ on a straight bond side perpendicular to the plate surface. Charpy tests and CTOD tests are carried out at 0°C and -40°C, respectively, where the CTOD test temperature is determined on the basis of WES2808 correlation formula. It has been found that the Charpy absorbed energy is affected by the chemical composition of the base steel, such as C, Mn, P, S, N, and Ti. Based on similar test results, an engineering parameter, fHAZ, is proposed by the Building Center of Japan for estimating the HAZ toughness. However, the present study indicates that the total amount of N is not a proper measure, but the free N controls the HAZ toughness. From the fracture mechanical point of view, the correlation between the Charpy energy and the critical CTOD is useful. It is noted that the CTOD test results are sensitive to the length of CGHAZ along the crack front. The Charpy-CTOD correlation with respect to the HAZ toughness should be made with attention to the base-metal chemical compositions as well as the CGHAZ size along the crack front.

Improvement of HAZ Toughness for High Heat Input Welding by using Boron Diffusion from Weld Metal

In high heat input welding, such as 1-pass electrogas arc welding (EGW) of thick steel plates, HAZ toughness is deteriorated because HAZ microstructure coarsens significantly. TiN precipitates dispersed in steel plates are effective to suppress coarsening HAZ microstructure by pinning effect. However TiN precipitates resolve in high temperature region near the fusion line, where deterioration of HAZ toughness is unavoidable near the fusion line. As a solution of this problem, the authors paid attention to boron in the weld metal. Boron is known to be effective to prevent coarse ferrite formation at prior austenite grain boundaries, and to diffuse rapidly in steel. In this research, boron diffusion from weld metal to HAZ during EGW process was confirmed by SIMS analysis. It was certified that diffused boron in HAZ improved HAZ toughness because boron suppressed microstructure coarsening, and fixed free nitrogen originated by TiN resolution as BN near the fusion line. These research results proposed a new HAZ structure control technology considering combination of high boron bearing weld metal and TiN-treated steel plates.

大入熱溶接熱影響部における島状マルテンサイトの形態と破壊靭性

A study was made on the relation between CTOD value and M-A constituent for the single-heat-cycled weld HAZ of YS 320 to 360 N/mm2 class TMCP steels with increased heat input. It was found that the M-A constituent disappeard and lost its deterioration effect on the HAZ CTOD toughness when the heat input exceeded about 15kJ/mm, although this boundary heat input depended on the steel chemistry. On the other hand, the austenite grain size increased monotonically with increasing the heat input. But the austenite grain size could not be the controlling factor of the HAZ toughness, and its effect deferred between base metals. However, the HAZ toughness was related to the fracture facet size for the large heat input conditions. This fracture facet size, which represents the fracture toughness, is considered to be a measure of the uniformity of the transformed microstructure from austenite in the HAZ.

780MPa級鋼の再現HAZにおける組織と靭性に及ぼすTiおよび合金元素の影響

780MPa級鋼の再現HAZにおける組織と靭性に及ぼすTiおよび合金元素の影響

Ti脱酸鋼の大入熱溶接相当再現HAZの低温靭性に及ぼすMnの影響

Ti脱酸鋼の大入熱溶接相当再現HAZの低温靭性に及ぼすMnの影響

低炭素ベイナイト鋼の再現HAZにおける組織,靭性に及ぼすボロンの影響

低炭素ベイナイト鋼の再現HAZにおける組織,靭性に及ぼすボロンの影響

Study of welding ultra-fine grained ferrite steel by CO2laser

Ultra-fine grained ferrite steels have higher strength and better toughness than the normal ferrite steels because of their micrometer or sub-micrometer sized grains. In this paper the ultra-fine grained steel SS400 is welded by CO 2 laser. The shape of weld, cooling rate of HAZ, width of HAZ, microstructures and mechanical properties of the joint are discussed. Experimental results indicate that laser beam welding can produce weld with a large ratio of depth to width. The cooling rate of HAZ of laser beam welding is fast, the growth of prior austenite grains of HAZ is limited, and the width of weld and HAZ is narrow. The microstructures of weld metal and coarse-grained HAZ of laser beam welding mainly consist of B L + M (small amount). With proper laser power and welding speed, good comprehensive mechanical properties can be acquired. The toughness of weld metal and coarse-grained HAZ are higher than that of base metal. There is no softened zone after laser beam welding. The tensile strength of a welded joint is higher than that of base metal. The welded joint has good bending ductility.

HAZ Toughness of Multipass Welded Joints: Influence of Brittle Zones

HAZ Toughness of Multipass Welded Joints: Influence of Brittle Zones

Effect of Local Melting on the HAZ Toughness of GMA Welded AA6082 and AA6005 Extrusions Containing Cu

Effect of Local Melting on the HAZ Toughness of GMA Welded AA6082 and AA6005 Extrusions Containing Cu

高窒素含有Niフリーオーステナイト系ステンレス鋼のHAZ靭性に及ぼす窒化物析出の影響

高窒素含有Niフリーオーステナイト系ステンレス鋼のHAZ靭性に及ぼす窒化物析出の影響

Accounting for constraint effects in fracture mechanics analysis of floating production, storage and off-loading vessels and ships

Research has been performed to study the effect of constraint dependent fracture toughness of parent metal and HAZ for steels applied for fabrication of oil and gas floating production, storage and off-loading (FPSO) vessels and ships. A test method was employed to study the HAZ crack tip opening displacement (CTOD) fracture toughness at various levels of constraint avoiding excessive scatter usually associated with conventional HAZ CTOD testing, which may obscure effects of constraint. The objective of the research was to determine the material dependent constraint parameters in Ainsworth and O'Dowd's constraint modified fracture assessment approach and to develop a method based on information from the literature pertaining to the structural constraint in plates with semi-elliptical surface cracks. Results due to Wang and Parks and Nakamura and Parks computed using line-spring and FE analyses were used to establish polynomial expressions for the structural constraint in tension and bending applicable to fracture assessment of shallow fatigue cracks initiated at weld toes in floating offshore structures such as FPSOs. The results of the analyses clearly show that fracture assessment of semi-elliptical surface cracks in tension would be overly conservative if constraint effects are not accounted for in the Option 1 or Option 2 fracture assessment curves or in fracture toughness. It was also found from the tests conducted on specially prepared wide-plate test specimens, that the constraint modified Option 1 curve was conservative for fracture assessment of semi-elliptical surface cracks located in the HAZ of a 500 MPa minimum specified yield strength quenched and tempered steel for offshore application.

An Appraisal on Assessment of HAZ Toughness

An Appraisal on Assessment of HAZ Toughness

The Effect of Vanadium and Niobium on the Properties and Microstructure of the Intercritically Reheated Coarse Grained Heat Affected Zone in Low Carbon Microalloyed Steels.

Four steels, C-Mn-0.05V, C-Mn-0.11V, C-Mn and C-Mn-0.03Nb, all essentially boron-free were subjected to processing to simulate the microstructure of a coarse grained heat affected zone (GC HAZ) and an intercritically reheated coarse grained HAZ (IC GC HAZ). This involved reheating to 1 350°C, rapid cooling (Dt8/524 s) to room temperature and then reheating to either 750°C or 800°C. The toughness of the simulated GC HAZ and IC GC HAZ was assessed using both Charpy and CTOD tests and the hardness of both zones was also measured. A detailed assessment of the size and area fraction of martensite-austenite (MA) phase in the IC GC HAZ in the steels was obtained from a combination of Scanning Electron Microscopy (SEM) and Image Analysis of the resultant SEM micrographs. In addition, the distribution of the M-A phase was examined by observing 250 fields at a magnification of 2 500 times in the SEM for each of the steels. It is clear that the alloying addition has a significant effect on the amount and size of the M-A phase. The addition of 0.05% V to the C-Mn steel resulted in the lowest IC GC HAZ Charpy 50J impact transition temperature and the 0.1 mm CTOD transition temperature. The corresponding size and area fraction of the M-A phase were the smallest of the four steels. Raising the level of vanadium to 0.11% caused a deterioration in IC GC HAZ toughness, which was reflected in a greater area fraction of M-A phase, larger mean and maximum sizes of M-A particles and significantly more fields containing M-A phase. The addition of 0.03%Nb produced poorer IC GC HAZ toughness data than C-Mn-V and C-Mn steels and this was related to the large size and area fraction of M-A phase quantified in the Nb steel. The presence of M-A phase is considered to be the dominant factor in determining the toughness of IC GC HAZ.

Investigation of dynamic fracture toughness (JId) behaviour of strength mis-matched Q & T steel weldments using instrumented Charpy impact testing

Abstract Instrumented Charpy impact testing has been used for evaluating the dynamic fracture behaviour of mis-matched Q & T steel weldments. The effect of strength mis-match on the dynamic fracture toughness (JId) and dynamic crack resistance curves (Jd–R) of HAZ and weld metals specimens are discussed. The crack propagation and mode of failure due to impact loading are analyzed through SEM and correlated with dynamic resistance curves.

溶接熱影響部粗粒域のＴｉＮ析出形態，微視組織ならびに靭性に及ぼすＴｉ含有量の影響

As reheating temperature of coarse grain HAZ is around 1623 K, TiN and austenite compositions are reported to reach equilibrium. This means that size and a volume fraction of the TiN precipitate could be predicted by thermodynamic data. As austenite grain size has close relation with average size and a volume fraction of TiN, prediction of the austenite grain size is also possible. In order to establish a prediction way of sizes of TiN and austenite grain of coarse grain HAZ, the relationship between size of the TiN and composition of the TiN that was calculated by thermodynamic data was investigated. The relation between the TiN precipitate morphology and size of the austenite was also examined. The effect of microstructure and the TiN size on coarse grain HAZ toughness of Ti containing steels was also investigated.The size of the TiN showed close relationship with chemistry of the TiN and the austenite that equilibrate each other. The austenite grain size showed to follow the Zener's relation with the TiN size and a volume fraction. These results show that the prediction of the austenite grain size of coarse grain HAZ is possible by calculating equilibrium chemistry of TiN and austenite.HAZ toughness of Ti containing steels reveals close relation with the grain size and the TiN size.

Weld HAZ embrittlement of Nb containing C–Mn steels

The adverse influence of Nb on weld HAZ properties is still an active issue of discussion between construction companies and steel manufacturers. Some controversy exists in the literature concerning the influence of Nb on HAZ properties under certain conditions, and this investigation was subsequently performed over a range of C and Nb compositions, typical of the steels concerned, and for three different single cycle heat inputs ranging from 1.5 to 6 kJ/mm. Simulated thermal cycles were employed, using a Gleeble 1500 thermomechanical simulator, followed by CVN testing. It is shown that Nb additions can have a detrimental or beneficial effect at low heat inputs, depending on the C level, but a severe detrimental effect of Nb on HAZ toughness is observed at high heat inputs and C levels.

Toughness investigation on simulated weld HAZs of SQV-2A pressure vessel steel

In order to get detailed information about weld HAZs toughness of SQV-2A steel and determine the optimum welding and heat treatment parameters, the toughness of simulated CGHAZs (coarse grained heat affected zone) and CGHAZs (intercritically reheated CGHAZ) were systematically investigated. The influence of tempering thermal cycles on weld ICCGHAZs toughness was clarified. The effect of post weld heat treatments (PWHT) on weld CGHAZs toughness was also determined. The results showed that high toughness (absorbed energy >200 J) of weld HAZs could be achieved by selecting the optimum welding and PWHT parameters (cooling time Δ t 8/5: 6–40 s, PWHT: 893 K, 3.6–7.2 ks). Tempering thermal cycles with peak temperature of above 573 K could remarkably improve the toughness of deteriorated ICCGHAZs and reduce the hardness, when cooling time Δ t 8/5(2) of the reheating thermal cycle was 6 s, which implies that welding of SQV-2A without PWHT is possible, provided that low heat input welding is adopted and welding procedure is correctly arranged. Metallography and fractography revealed that M–A constituents in weld HAZs played an important role in controlling weld HAZ toughness.

建築構造用電炉厚板の大入熱溶接熱影響部の靱性

We have examined the characteristics of welded joint part in order to improve the toughness of HAZ. The probe samples used in this study are the steel plates produced by EAF-Plate mill process bearing Titanium for building. The remarkable improvement of toughness in HAZ is confirmed in the case of ultra large heat input weld condition. It is proved that sufficient HAZ characteristics can be attained by adjust the prompt analysis condition even in large heat input weld condition with 60-90kJ/mm.